In magnetic bubble domain technology, magnetic bubble domains are used to represent information. For example, the presence of a magnetic bubble domain can be a binary one, and the absence of a magnetic domain can be a binary zero. Additionally, the bubble domains can be coded in accordance with their wall magnetization properties so that one type of domain can represent a binary one, while another type of domain represents a binary zero. Still further, multi-level coding is possible.
In typical magnetic bubble domain memories, bubble domains are moved in shift register patterns in order to store information. Various techniques have been shown for movement of bubble domains. In one of these techniques, so called "contiguous disk" devices are used in which the propagation elements for moving the bubble domains are contiguous with one another. This type of propagation pattern has particular advantages in terms of the density which can be achieved for a given lithography. Since the propagation elements themselves are considerably larger than the bubble domains, the lithographic constraints on the techniques used to make the contiguous propagation elements are minimized for a given size of bubble domain.
Contiguous disk type bubble domain structures can have shapes other than those involving circles. For instance, contiguous diamond patterns have been shown, and in general the propagation pattern is one in which a bubble domain is moved along a generally undulating pattern edge, there being convex and concave portions alternately arranged along the edge. These contiguous element propagation patterns have typically been fabricated by ion implantation of a magnetic garnet layer. Ion implantation causes the magnetization within the garnet to lie substantially in the plane of the garnet and, if certain material constraints are followed, the ion implanted garnet will support stable magnetic charged walls. These magnetic charged walls have been described by Y. S. Lin et al in IEEE Transactions on Magnetics, Vol. MAG-14, No. 5, September 1978, at Page 494.
The use of magnetic charged walls to move magnetic bubble domains coupled to these walls has been described by G. S. Almasi et al in American Institute of Physics Conference Proceedings, Vol. 24, Page 630, (1974) and by Y. S. Lin et al, J. Appl. Phys., Vol. 48, Page 5201 (December 1977).
In contiguous element propagation patterns of this type, the existence of stable magnetic charged walls is essential for movement of the bubble domains. U.S. Pat. Nos. 4,070,658 (propagation), 4,142,250 (translation switch), and 4,128,895 (nucleator) describe various devices using magnetic charged walls for movement of bubble domains. Although a complete magnetic bubble domain chip can be devised using magnetic charged walls to perform many of the storage functions, the use of magnetic garnets to provide the ion implanted drive layer is not without some disadvantages. For example, ion implanted magnetic garnet films exhibit cubic anisotropy in the plane of the drive films. This means that in-plane easy and hard axes are present, and larger magnetic drive fields have to be provided to move the bubble domains from an easy axis direction toward a hard axis direction. In order to overcome this, co-pending application Ser. No. 959,960, filed Nov. 13, 1978, in the name of C. D. Cullum et al (now U.S. Pat. No. 4,247,912) describes the design of a magnetic bubble domain chip in which the various components comprising the chip are aligned in specific directions with respect to the easy axis directions in the plane of the garnet drive layer.
In order to provide good magnetic coupling between a magnetic charged wall and a bubble domain which is to be moved by the charged wall, certain considerations have to be followed with respect to the magnetization and thickness of the drive layer, and with respect to the magnetization and thickness of the bubble domain film. As a general rule, the thickness-magnetization product of the drive layer must be approximately the same as that of the bubble domain storage layer for good coupling. As the magnetic bubble domain size decreases, the magnetization of the storage layer must increase. In order to provide sufficient charged wall coupling, the magnetization of the drive layer would also have to increase. However, as the magnetizaton of the drive layer increases, it becomes more and more difficult to successfully ion implant it to provide an in-plane drive layer. Thus, for bubble domains of approximately 1 micron and less, the use of garnet drive layers may not be possible. As the bubble domain size decreases to approximately 1/2 micron, the use of garnet drive films for contiguous disk type devices will be very difficult.
Another factor with respect to the use of garnet drive layers is that there is not a great deal of flexibility in their preparation, and the amount of magnetization 4.pi.M.sub.s which can be obtained is limited, the maximum being about 1750 G for Yttrium Iron Garnet. Thus, it is difficult to increase the thickness-magnetization product in garnets without using very thick films. At the present time, they are primarily grown by liquid phase epitaxy or by chemical vapor deposition. Although some work has been described with respect to evaporation and sputtering of garnet films, well defined techniques for reproducible growth of these films is not clearly evident. However, as the magnetization increases, the difficulty in ion implantation also increases, as noted previously.
In the practice of the present invention, another suitable drive layer having particular utility for the provision of stable magnetic charged walls for movement of very small bubble domains has been sought. For this purpose, a class of amorphous magnetic materials is proposed, the materials being ferrimagnets having an in-plane magnetization with zero, or very little planar anisotropy. These amorphous ferrimagnets have low coercivity, a hard perpendicular anisotropy, and a magnetization which lies perfectly in the plane of the drive layer. This contrasts with the magnetization in the plane of an ion implanted garnet layer, which generally makes a very small angle with respect to the plane of the drive layer. Due to this canted magnetization in the garnet drive layers, the propagation magnetic field has to overcome this small vertical angle, in addition to having to overcome the hard in-plane axes in the garnet layers.
Amorphous ferrimagnetic drive layers have advantages over garnet drive layers in that they do not exhibit a cubic anisotropy in the plane of the film and therefore do not have the in-plane orientation effects described with respect to the garnets. For this reason, potentially lower magnetic drive fields are required and the propagation patterns do not have to be aligned with respect to certain axes. Also, the amorphous ferrimagnetic films can be prepared in a number of ways, including sputtering and evaporation, and have a wide range of properties because their compositions can be varied over greater ranges than those of the garnets. For example, alloy metallic films can be prepared having high 4.pi.M and adjustable Q (quality factor of the film). In this manner, they are more useful as drive layers for small bubble domains which require high 4.pi.M materials. Since ion implantation is not required to produce the necessary in-plane magnetization, these amorphous drive layers can easily be used with bubble domain storage layers having high magnetization.
The choice of ferrimagnetic amorphous materials as drive layers in contiguous disk type devices involved considerable experimentation and was not apparent from the prior art. A substantial body of prior art existed which suggested that charged magnetic walls would not be stable in a film without in-plane anisotropy H.sub.k. For example, Puchalska et al compared the domain structure of ion implanted garnets to that of amorphous Co-P, among others, at the Magnetism and Magnetic Materials Conference, Cleveland, 1978, and concluded that they had different types of domain structure. That is, Co-P did not support charged domain walls. At this same conference, Kobliska et al reported on the magnetic properties of amorphous metalloid compositions, such as FeB. They suggested that these compositions could be used as a replacement for permalloy in bubble domain devices such as C-bar devices. No mention was made of their applicability as replacements for garnets in contiguous disk type devices, or that they would be able to support charged walls. At the 1979 Intermag Conference, held in Florence, Italy, C. C. Shir et al indicated that the cubic anisotropy in ion implanted garnets was essential for charged wall stability. Further, H. Callen, J. Appl. Phys. 50, No. 3, Page 1457 (March 1979) indicated that an odd-fold planar anisotropy was essential to bubble propagation by ion implanted garnet layers. Since suitable amorphous films can be made which do not exhibit this anisotropy, and since amorphous films having very small planar anisotropy have two-fold (even) anisotropy, this art led away from their possible use as charged wall drive layers.
Accordingly, it is a primary object of the present invention to provide contiguous propagation element bubble domain devices using an improved drive layer capable of supporting stable magnetic charged walls which can be used to move magnetic bubble domains having diameters of one micron and less.
It is another object of the present invention to provide magnetic charged wall bubble domain devices using amorphous magnetic layers as drive layers.
It is another object of the present invention to provide magnetic bubble domain devices using magnetic charged walls to move the bubble domains, which require smaller drive magnetic fields than those presently used.
It is still another object of the present invention to provide an improved drive layer for supporting magnetic charged walls for movement of magnetic bubble domains, wherein the improved drive layer has very small, or substantially zero anisotropy in the plane of the drive layer.
It is a further object of the present invention to provide a magnetic drive layer for movement of magnetic bubble domains by charged walls, wherein the drive layer can be easily fabricated with a wide choice of magnetic properties.
It is a still further object of the present invention to provide magnetic bubble domain devices using magnetic charged walls for movement of magnetic bubble domains, wherein the drive layer supporting the charged magnetic walls can be fabricated by a variety of methods, and on any type of magnetic bubble domain storage layer.
It is a further object of the present invention to provide an amorphous drive layer capable of supporting stable magnetic charged walls, in which the magnetic charged walls will move smoothly during reorientation of the magnetic drive field, in contrast with the movement of magnetic charged walls in garnet drive layers, which is often uneven due to "whip and flip" charged wall motion.
It is another object of the present invention to provide amorphous ferrimagnetic metals which can support stable and mobile charged walls in layers having very small in-plane anisotropy field.